All but One Dry Dam Eliminated from Kananaskis Country Flood Mitigation Planning

At the recent Flood Symposium in Calgary the Alberta Government a short list of flood mitigation projects that they would implement over the next three years. The good news for Kananaskis Country is that all of the previously-proposed dry dams have been eliminated, though one new one has been proposed on the Elbow River just above the confluence with McLean Creek. There are three options for reducing flow on the Elbow: the 58 St diversion tunnel, an off-stream diversion reservoir in the Springbank Area and the McLean dry dam. The was some suggestion that only two of these three projects would eventually be built. A decision on this project will be made after the Calgary underground diversion feasibility study is complete. So it may never be built. However, if it’s not built, other works such as dykes or berms will be required to protect Bragg Creek.

McLean Dry Dam

Artists rendering of the proposed McLean dry dam. Picture ESRD.

The proposed dam is 50 m high, with a storage capacity of 49 million cubic metres. It would require relocation of a portion of Highway 66 and the highway’s bridge over the Elbow River. As you can see from the map below there would be significant changes in the Allen Bill Pond area, with a new start to Elbow/Fullerton trails from a new recreation area. As none of this is likely to happen before 2017, we are going to have to live with temporary trails and trailheads until a decision is made and a master plan for the area trails can be developed.

Alf Skrastins has come up with a different interpretation of the artists rendering that is an improvement on my initial, hurried thoughts. Based on his ideas the area subject to flooding is much larger. Here is a revised map.

McLean Dry Dam map

Showing the area likely to be flooded when water backs up behind the dry dam.

There was no indication of the flow level at which the dry dam would be activated. For example, will water start to back up behind the dam if a 2005 level flood was expected? If so, we can expect some flooding upstream every 8-10 years. Is this environmental acceptable?

For more information on dry dams see my previous blog Flood Control Structures Considered for Kananaskis Country.

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13 comments… add one
  • Neil Winter Jul 20, 2014, 10:22 pm

    Building the flood diversion tunnel from Glenmore resevoir to the Bow,would help folks in Mission etc, but the Bow flooded Inglewood and Bowness, what about that?
    Seems like a lot of money wasted for a once in 150 year event.
    But then I live in Shepard, with no river in site.

  • Dave Mayhood May 8, 2014, 7:42 pm

    With regard to the feasibility of moving whole communities off floodplains, not just a few buildings, I will just say that it has been done before. Rapid City SD (pop. ~40,000) moved all housing and most commercial buildings off the floodplain after a 1970s flood killed about 230 people. I am sure that there are others (High River is doing it with 1 neighbourhood). Take Bragg Creek, for example. The province owns enough land very close by in K Country. Alberta could relocate the village to the terrace above the river just inside the KC boundary, and reclaim the floodplain part of Bragg Creek as park land, its most appropriate use. Properties can simply exchanged, and the buildings bought at fair market value, moved or rebuilt at the new site. Keep in mind that the estimated replacement costs of the 2013 flood were in the several billions of dollars. We are going to spend that money anyway, so we might as well solve the problem once. Dams are not permanent even with ongoing maintenance, but our communities need to be permanent. We are going to be here for 100s to 1000s of years. Do it right once and be done with it. I could make a similar case for most development on the Bow R floodplain at Calgary, but I think the above gives some idea of how it would work, it would just need to be done over decades.

  • Tyler May 8, 2014, 11:36 am

    Alf, again I agree with you. Not for the sake of avoiding argument, but once again you are correct . The solution I propose can’t be done anymore because you hit the nail on the head with having to move every home, business, building well away from the waters edge. We all know this is logistically not possible. Generations ago, maybe but you would have to go a long way back in the history of human settlement to do so. If this structure is built correctly and does what it is intended for, I ok with it, like I said before, One is a lot better than the few others that were planned on other waterways.

  • Mark Giesbrecht May 5, 2014, 8:34 pm

    Hey Gents,

    I was looking for some info on how the extent of coverage was calculated in the map above; if you’d like, contact me and I could provide some elevation models and an approach to calculate the fill of the dam.

    Cheers,
    Mark

  • Tony Daffern May 4, 2014, 2:45 pm

    Thanks for the input Dave. I don’t think the dam would have to be built of concrete. Large earth dams, properly constructed and maintained have proved to be reliable. Failure would most likely occur if the water level rose to where it overtopped the dam. This is normally dealt with by a spillway. As Kevin pointed out, debris will be a big problem. The upstream spillway openings would have to allow debris to pass through to pile up in the lower river and the mess cleaned up later. In view of the uncertainty of future maximum flows the dam would probably need an additional emergency spillway that would spill water into the valley below probably taking out the highway, but much better than having the dam fail, releasing 49 M cubic metres of water in a few hours. A dam so close to Bragg Creek cannot be allowed to fail. The consequences are unpredictable and unthinkable.

  • Dave Mayhood May 4, 2014, 11:29 am

    I want to pick up on a couple of the ideas discussed by Tony and Kevin, and develop them further for the sake of argument.
    (1) As Tony noted, this dam can never be allowed to fail because of the catastrophic consequences if it did. This means that it must be designed to withstand the maximum possible flood. I wonder what that flood size would be, and how it would be determined in a warming world in which flood frequency may be increasing. It seems to me that such a dam would have to be massive, and would not be built of rock and dirt, which are erodible, but of concrete, a very expensive structure. Whatever the case, it would likely be much larger than the structure presently being considered, and would require maintenance and successive replacement, forever — essentially the cost is unlimited. (And by the way, dam costs are almost always seriously underestimated by their proponents.)
    (2) Most people seriously underestimate frequencies of unlikely events over even moderately short periods, so also underestimate the risk that they will experience severe floods. This is an issue because Alberta as a responsible society needs to plan our towns, cities and countryside to be livable effectively in perpetuity. (How many of us, when you think of it, intend to just loot the province and then leave, taking all of our descendants with us?)
    Much critical infrastructure is designed to withstand 1:100-year events. That is, there is a probability of 0.01 (1%) that an event of that size will occur in any given year. What is too often overlooked is that people don’t live in houses or work in factories or offices for just 1 year; people (not necessarily the same ones) will be there for as long as the community exists. Let’s say people will be living on the floodplain below this dam for 200 years — a severe underestimate, I suspect. If the proposed dam is built for the 1:100-year event, the probability that the people of Bragg Creek and Calgary will experience a catastrophic, lethal flood is a little less than 87%. More likely the design criteria are higher — for a 1:300-year event, say. In that case, the likelihood of many Albertans dying from a flood that takes out the structure is a little less than 49% over a 200-year period. Even for a dam designed to withstand a 1:500-year flood, the probability of catastrophic, lethal flooding downstream is nearly 33% within 200 years.
    If the floodplains at Bragg Creek and Calgary are occupied for a more realistic period, say 500 years, the probability of a catastrophic lethal flood increases substantially. For a dam properly designed and maintained to withstand a 1:500-year flood, there is a probability of a little less than 63% of a catastrophic flood in those communities within 500 years.
    These figures apply to dams adequately constructed and maintained for the design floods mentioned. Kevin pointed to realistic mechanisms that could easily cause a flood-control dam of this type to clog and fail, with lethal consequences, no matter how well-built. Short of denuding the upper Elbow watershed of most of its riparian forest, it is difficult to see how the essential design problem could be overcome. Trash racks at intervals along the entire upper river?
    To my mind, it makes far more sense to treat flooding as a land-use problem rather than a water management problem. It’s both cheaper and safer. Sound public policy would assist people to move off of floodplains to higher ground. It is a permanent solution to what is currently a perpetual, lethal problem that is only made worse by flood-control dams.

  • Kevin VT May 3, 2014, 11:23 pm

    Alf, the debris problem is not the woody debris that accumulates on the floodplain between major flood events. It is the massive amount of washed-out pine and spruce that would come down in real time, during the major flood event, especially once the reservoir behind the dry dam begins to fill. The saturated valley slopes combined with the shallow rooting depth of spruce and pine and the massive hydraulics at work during the flood will put immense amounts of woody debris into the river when it is at its most unapproachable state. When (not if) it accumulates at the constriction in the dry dam, it will start a chain-effect that further constricts the flow, both increasing sediment deposition upstream and the scouring garden-hose effect downstream. Tony is right that a cement spillway will be needed, but if it clogs with washed-out forest, it will be worse than useless. The cobbles you describe become mobile during a big flood. That’s how they got there in the first place. Constrict the floodplain, and they are no longer as mobile. A dry dam constricts the floodplain during big floods – a dam clogged with flotsam does so to an even greater degree. Moving Bragg Creek and Redwood Meadows homes out of the river (i.e. off the floodplain) may seem an undesirable alternative to a magic engineering solution, but you can be darn sure that when a dry dam blows out with a full reservoir, those homes will get moved.

  • Tony Daffern May 2, 2014, 2:01 pm

    You could be right Alf. The picture is very much an artists impression, and there are a number of things that don’t make sense; for instance the highway doesn’t curve like that. I had assumed that the stream coming in from the left was intended to show McLean Creek. We will just have to wait until some more specific drawings are released.

  • Alf Skrastins May 2, 2014, 12:07 pm

    The regular course of the Elbow is that curved channel near Highway #66. The spillway is the straight channel that comes over the low ridge where Snagmore trail currently goes.

  • Tony Daffern May 2, 2014, 10:20 am

    The construction of the McLean dry dam would require a significant amount of fill, and I wonder where it would come from. While local material could form the bulk of the dam, material such as clay would have to be found to provide an impervious core. Such a core is needed because the rapid filling and draw-down in the event of extreme flooding would result in a large difference in pore pressure between the upstream and downstream faces of the dam. I am told that without an impervious core, failure usually starts by the downstream face turning to slurry, followed by a rapid collapse of the whole structure.

  • Tony Daffern May 2, 2014, 10:12 am

    One thing that is missing on the artist’s rendering is a spillway. This dam cannot be allowed to fail. Ever! The potential for loss of life and extensive damage to Bragg Creek would be enormous. This dam will require a spillway large enough to take the full flood flow in event that the reservoir fills up before the storm finished. In this mornings Herald there was an article about the Bassano Dam coming close to failure because it’s spillway was not large enough. Even though there was little danger of loss of life in this case, the economic consequences would have been significant.

  • Alf Skrastins May 1, 2014, 9:41 pm

    If this is not the solution that is needed… can you suggest a better solution? The proposed structure is a dry dam. Emphasis on the word DRY. For 365 day in most normal years, the Elbow River would flow unimpeded through the “culvert” in the dam and there would be no reservoir backed up behind it. In the event of a catastrophic flood event, the water in excess of some yet to be announced flow level would be held back behind the dam. This would happen perhaps every 5, 10 or 20 years, depending on the flood flow level that is determined to be acceptable. The water would still pour out of the opening at an acceptably high flow rate and the temporary lake would drain itself dry in a short time period after the flood causing rain/melt event stops. Any logs or debris that was carried down from the upper Elbow River would settle out and litter the dry floor of the temporary lake. It is my understanding that this would be cleaned up each year, so that debris does not accumulate.
    I suggest walking along that section of the Elbow and looking at it yourself. Right now it is mostly a wide, dry cobble flat, filled with scattered debris from the 2013 flood. It’s not like we’re talking about prime habitat or recreation areas here.
    I’m not saying I am really happy with this option, but the other option would be to move Bragg Creek and the homes and businesses between Glenmore Dam and downtown Calgary.
    It appears that Highway #66 would be re-build on the south side of the Elbow River, past McLean Creek campsite and it would re-join the current Highway #66 somewhere around Paddys Flat campground.
    From a recreation trail users perspective, the impact would be a re-thinking of the trailhead locations and trail routing in the area around Allen Bill Parking Lot and the mess of buildings and junk that make up the Elbow Administration compound. That area could use a good make-over anyhow!

  • Tyler May 1, 2014, 5:08 pm

    I have to admit, I am a bit conflicted, although one dam is a lot better than the three or four that were originally discussed on other area rivers in terms of environmental impact, I still don’t think it is the solution that is needed. That being said, Tony brings up a valid point in terms of the impact upstream of the dam. Is it acceptable? Absolutely not, but given what could have been with what was originally planned with dams on the Highwood etc etc. it is a lot less than if Plan A would have went ahead. I am curious as to how Highway 66 will be rerouted. Would we see the obvious realignment to the northwest of where it currently sits? Or could we possibly see a similar situation like on Highway 3 over the Oldman Dam and go right over top of it? Albiet it is fairly narrow where it crosses .

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